Often, prime movers are used to provide motive power to a generator which in turn converts the motive power into electrical power for energizing loads. Typically, the prime mover is started by a dedicated starter motor or by operating the generator itself as an induction motor to develop the necessary torque level for starting the prime mover. However, the former approach unduly increases system size and weight while the latter gives rise to large transients in the AC power which must be supplied to the generator to cause it to operate as a motor. The magnitude of the transient may be such as to render portable power supplies, such as ground power carts, unsuitable for this use.
Fletcher et al U.S. Pat. No. 3,867,677 discloses a starting system for a synchronous motor which drives a generator. An induction starting motor is provided AC power to accelerate the rotors of the synchronous motor and the generator. Electrical power is thereafter supplied to the synchronous motor to cause it to supply motive power to the generator. By rotating the synchronous motor before applying power thereto, large power line disturbances are said to be prevented.
Fletcher et al also discloses a circuit for operating the motor at unity power factor. A phase comparator compares the phase of the input voltage to the phase of the input current to develop an error signal which controls the motor field to keep the motor at unity power factor.
The Fletcher et al system, however, cannot reduce power line disturbances to a great extent since the synchronous motor and generator rotors are both accelerated by the induction starting motor prior to the application of electrical power to the synchronous motor. This high-inertia load on the induction starting motor results in a significant transient at the time the AC power is supplied to the induction motor.
Mehl et al U.S. Pat. No. 4,481,459, assigned to the assignee of the instant application, discloses a starting/generating system and method wherein a brushless generator is connected by a torque converter to a prime mover. When it is desired to start the prime mover, a permanent magnet generator of the brushless type is provided electrical power to bring the rotor of the brushless generator up to a predetermined speed. Once this predetermined speed is reached, the main generator windings of the brushless generator are provided power to operate the brushless generator as a motor and thereby develop motive power. The torque converter is then actuated so that the motive power is returned through the torque converter to the prime mover to bring it up to self-sustaining speed.
Cronin U.S. Pat. No. 4,473,752 discloses a starter-generator machine for starting an aircraft engine. The machine includes a rotor-shaped stator which is fixed within a squirrel-cage induction rotor. The induction rotor in turn includes an array of magnets attached on the outer circumference thereof. The machine is operated as a starter by applying three-phase AC power to windings disposed within the rotor-shaped stator. This in turn accelerates the induction rotor and the permanent magnets. Once a predetermined rotor speed is reached, AC power is applied to stator windings in an outside stator surrounding the rotor to synchronize the rotating magnetic field developed by the permanent magnets with the rotating field created in the stator so that motive power is developed. The motive power is transferred to a prime mover to start same.